1. Field of the Invention
The present invention relates to a radiation-detecting light-transmission apparatus for use in nuclear power plants, radiological facilities, and the like which deal with radiation and radioactive substances.
2. Description of the Related Art
Scintillation detectors, semiconductor detectors, and the like have been used for detection of radiation and measurement of radiation in nuclear power plants, radiological facilities, non-destructive inspection technology, particle accelerating facilities, cyclotron radiation facilities, and so forth in which radioactive substances have to be handled. It should be noted that the term "radiation" used herein means alpha rays, beta rays, gamma rays, neutron rays, and X rays, individually or collectively.
In such scintillation detectors, a detecting section is composed of a scintillator and a photomultiplier tube assembled together, while in the semiconductor detectors, the detecting section is composed of a semiconductor sensor, high voltage power circuits, and signal circuits combined with one another. Detection signals are sent to a counting section through the signal circuit. The detection signals are analyzed by a single channel analyzer with count rates. Alternatively, they undergo spectral analysis treatment by a multi-channel wave height analyzer.
When a detecting and counting section are desired to be separated from each other in the conventional detectors before mentioned, a power cable and an electric signal cable are used therebetween. In place of the electric signal cable, it has been recently proposed that electric signals are converted into light signals at the detecting section and thereafter the light signals are transmitted through an optical fiber cable. In either of such systems, the detecting section has a power supply, and hence some countermeasures against noise problems are needed. In an improvement for alleviating noise problems, it has been attempted that a scintillator and a photomultiplier tube, which are usually placed adjacent to each other, are spaced apart from each other and connected by a bundle of optical fibers or an optical pipe, and that a power supply is removed from the detecting section, and the light from the scintillator is transmitted directly to the photomultiplier tube without conversion. However, such an attempt has not been successfully employed in commercial applications.
Also, in Japanese Patent Application No. 1-336296 filed by the same applicants on Dec. 27, 1989 (Japanese Laid-Open Patent Publication No. 3-242590), a microlens is employed to connect an end of one or more fluorescent optical fibers of a radiation induced light wavelength shifter with one optical fiber. In such structure, the wavelengths of the light emitted by the scintillator are shifted by the fluorescent optical fiber, and the output light of the fluorescent optical fiber is optically transmitted for a long distance to the counting section which is spaced apart therefrom.
As described above, conventional devices or apparatuses such as the detecting section of the scintillation detector and the semiconductor detector need power supplies in their detecting section and signals are converted electronically therein. Thus, under adverse environmental conditions, such as high temperature atmosphere or strong magnetic fields, it happen that the photomultiplier tube and the semiconductor do not work properly and measuring operation is adversely affected.
To overcome such a problem, a design in which the detecting section and the counting section are sufficiently spaced apart from each other with a long cable may be implemented. Such design, however, includes a long signal line exposed to a surrounding electromagnetic field and/or induction noise, thereby requiring another countermeasure.
Incidentally, when the conventional apparatuses are used in water, they require a waterproof structure and careful handling since the detecting sections have high voltage power supplies. Accordingly, a detector which does not require such a waterproof structure, a long electric cable, or a power supply in the detecting portion has been needed for long time.
In the invention in the above-mentioned patent application which invention was proposed so as to solve such problems, a radiation induced light wavelength shifter 10, as shown in FIG. 14, comprises a cylindrical or columnar scintillator 11, reflecting sheets 12a and 12b, and a transparent layer 13. When radiation enters the scintillator 11, it emits light. The optical reflecting sheets 12a and 12b are disposed on the upper and lower surface of the scintillator 11, respectively. The transparent layer 13 is disposed on the outer periphery of the scintillator 11.
Fluorescent optical fibers 14a-14c are wound around and on the outer periphery of the transparent layer 13 disposed on the scintillator 11. The fluorescent optical fibers 14a-14c are housed together with the scintillator, in a reflecting casing 17. The fluorescent optical fibers 14a-14c are, at one end, optically connected to optical transmission fibers 15a-15c through microlenses 16a-16c, respectively.
The object of winding the fluorescent optical fibers 14a-14c around the outer periphery of the scintillator 11 is to increase the incidence area for light, namely, the amount of incident light into the optical fibers, because the amount of light entering the fluorescent optical fibers 14a-14c is proportional to the incidence area through which the light passes.
Another problem has been found. Winding of the fluorescent optical fibers 14a-14c around the cylindrical scintillator 11 tends to adversely affect the refractive index of the fluorescent optical fibers 14a-14c. In addition, a relatively larger amount of light leaks out of the bent fluorescent optical fibers 14a-14c themselves and at the connection portions to the microlenses 16a-16c. Besides, the focus of the microlenses 16a-16c is inclined to deviate from the respective optical axes of the optical transmission fibers 15a-15c. Consequently, transmission loss is relatively large. Moreover, since light emitted by the scintillator 11 only enters the inner half side of the fluorescent optical fibers 14a-14c which are wound around the scintillator 11, the fluorescent optical fibers 14a-14c cannot efficiently shift the wavelengths of all the light, emitted by the scintillator 11, thereby providing an insufficient amount of light shifted in wavelength.
Moreover, winding the fluorescent optical fibers 14a-14c around the scintillator 11 with large contact length needs increases the size of the scintillator 11. Furthermore, when the microlenses 16a-16c are used, it is rather impossible to individually place the focus of the microlenses 16a-16c in complete alignment with the optical axes of the optical transmission fibers 15a-15c. In this case, it is also very difficult to connect or disconnect the optical transmission fibers 15a-15c as transmission paths to or from the radiation induced light wavelength shifter 10.